api: add Anthropic Messages API compatibility layer

Add middleware to support the Anthropic Messages API format at /v1/messages.
This enables tools like Claude Code to work with Ollama models through the
Anthropic API interface.

Features:
- Request/response transformation between Anthropic and internal formats
- Streaming support with SSE events (message_start, content_block_delta, etc.)
- Tool calling support (tool_use and tool_result content blocks)
- Thinking/extended thinking block support
- Image content block support (base64)
- System prompt handling
- Multi-turn conversation support
- Proper stop_reason mapping (end_turn, max_tokens, tool_use)
- Error responses in Anthropic format

New files:
- anthropic/anthropic.go: Types and transformation functions
- middleware/anthropic.go: Request/response middleware

server/routes.go

@@ -1544,6 +1544,9 @@ func (s *Server) GenerateRoutes(rc *ollama.Registry) (http.Handler, error) {
 	r.GET("/v1/models/:model", middleware.RetrieveMiddleware(), s.ShowHandler)
 	r.POST("/v1/responses", middleware.ResponsesMiddleware(), s.ChatHandler)
 
+	// Inference (Anthropic compatibility)
+	r.POST("/v1/messages", middleware.AnthropicMessagesMiddleware(), s.ChatHandler)
+
 	if rc != nil {
 		// wrap old with new
 		rs := &registry.Local{